This invention relates to a method and apparatus for separating air. In particular, it relates to production of both a first oxygen product typically of normal purity and a second particularly pure oxygen product containing less than 100 volumes per million of argon impurity, and preferably less than 1 volume per million of all impurities.
One conventional method of separating oxygen from air comprises purifying the air by removal of water vapour and carbon dioxide impurities, cooling the purified air to a temperature suitable for its separation by cryogenic rectification, and subjecting the cooled air to rectification in a double rectification column comprising a higher pressure rectification column and a lower pressure rectification column. Typically, the top of the higher pressure rectification column exchanges heat with the bottom of the lower pressure rectification column so as to condense nitrogen separated in the higher pressure rectification column and reboil liquid oxygen separated in the lower pressure column. The lower pressure column typically has a bottom section in which argon is separated from oxygen. It is therefore possible to produce an oxygen product containing less than 3% by volume of argon. Indeed, no difficulty presents itself in producing an oxygen product containing no more than 0.1% by volume of argon. If, however, an oxygen product of substantially higher purity is required, there is a need to use one or more additional rectification or fractionation columns in order to remove impurities from an oxygen-containing stream withdrawn from the lower pressure rectification column. Not only may there be a need to remove impurities such as argon which are more volatile than oxygen, there may also be a need to remove impurities such as methane which are less volatile.
U.S. Pat. No. 5,049,173 discloses taking a feed stream from a region of the lower pressure column where the oxygen concentration is in the range of 1-35% by volume and stripping argon and other low volatility impurities from the stream in a side column. By taking the feed stream from a region of the lower pressure column where the oxygen concentration is in the range of 1 to 35% by volume, the concentration of impurities of relatively low volatility, for example, methane, in the feed stream is kept to a minimum. It is therefore possible to obtain a liquid oxygen product from the side column containing less than 1 volume per million of impurities in total. One disadvantage of this process is that a relatively large number of theoretical stages is required in the side column. In one example, approximately 64 stages are used. Another disadvantage is that the maximum production of high purity oxygen is in a typical example limited to 19% of the total oxygen production. A yet further disadvantage is that if the low pressure column is required to separate a stream of liquid air in addition to an at least partially vaporised fraction withdrawn from the bottom of the higher pressure rectification column, the feed to the side column contains less oxygen and therefore the total proportion of the oxygen products that can be produced at high purity is reduced.
U.S. Pat. No. 4,560,397 discloses a process in which the sole oxygen product is of high purity, containing, in one example, 10 ppm of argon, 1.3 ppm of krypton and 8 ppm of methane. A primary, higher pressure, rectification column and a secondary, lower pressure, rectification column are employed. An oxygen-enriched stream may be withdrawn from the primary column a few trays above the bottom tray so as to ensure that it contains a smaller concentration of impurities less volatile than oxygen than would be the case were it to be withdrawn from the bottom of the primary column. The oxygen-enriched stream is passed to the top of the secondary column which removes the argon impurity. A vaporous high purity oxygen stream is withdrawn from the secondary column at a point at least one theoretical tray above the bottom of this column. The secondary column is provided with a reboiler which is heated by nitrogen separated in the primary column. The nitrogen is thus condensed and is returned to the primary column so as to provide reflux for this column. However, in order to provide adequate reflux for the primary column, it is necessary to provide an additional means for condensing the nitrogen. A second condenser is therefore provided. This secondary condenser is cooled by a stream of oxygen-enriched liquid withdrawn from the bottom of the primary column. The resulting oxygen-enriched vapour is warmed by indirect heat exchange with the incoming air, is expanded in a turbine to provide refrigeration for the process, and is then rewarmed to ambient temperature by indirect heat exchange with the incoming air. As a result, the maximum yield of high purity oxygen that can be obtained is considerably reduced since a considerable proportion of the incoming oxygen is effectively vented from the process in the stream which is rewarmed.